The present invention relates to the production of sodium and potassium hydroxide and chlorine from sodium or potassium chloride, and especially to processes employing diaphragm or membrane cells for an electrolytic process therefore.
In the electrolytic production of alkali metal hydroxides (sodium hydroxide and potassium hydroxide, especially, are produced by electrolytic techniques), the net reaction is EQU 2MCl+2H.sub.2 O.fwdarw.2MOH+Cl.sub.2 +H.sub.2
with chlorine and hydrogen gases, respectively removed from the anode and cathode compartments as gases. Mercury cell processes for this reaction are being supplanted by diaphragm and membrane processes, with two electrolytic chambers separated by, respectively, a diaphragm or membrane. In both membrane and diaphragm processes, aqueous salt solution is fed into the anode chamber and the product MOH is removed from the cathode chamber. The processes differ in that the diaphragm (e.g. asbestos) permits MCl to migrate into the cathode and thus become mixed with the product MOH, while the membrane (e.g. perfluorosulfonic acid resin) permits only sodium or potassium cations (and hydrogen ions) to migrate to the cathode chamber, causing the depleted brine to be removed separately from the anode chamber.
Hydroxide formed in the cathode chamber of both diaphragm and membrane processes has a tendency to migrate, at least in part, back through the diaphragm or membrane, to the anode. If unneutralized, hydroxide will cause a series of reactions leading to hypochlorite, chlorate and/or oxygen. These reactions are generally avoided by adding HCl to the brine feed to the cathode, thereby maintaining a pH in the cathode of between about 0.5 and 6. Such acid may be formed by reacting product hydrogen with product chlorine, or as by-product of various processes employing chlorine. Both sources of HCl have disadvantages for many plants. For instance, the local need for hydrogen and chlorine may be sufficient to consume all of these gases that are produced. By-product HCl often has impurities or has its own local demand (e.g. for iron pickling or oxychlorination or hydrochlorination). In other plants, the by-product HCl is not generated locally, and must be purchased and transported back to the electrolytic plant.
Devices having stacked pairs of membranes have been suggested for various applications such as desalinization (U.S. Pat. No. 3,654,125 to Leitz) and springing of sulfur dioxide from aqueous sulfite and bisulfite solutions (U.S. Pat. No. 4,082,835 to Chlanda et al.), but such devices have no readily apparent application to the production of alkali metal hydroxides and chlorine.
One disadvantage of electrolytic cells for production of sodium hydroxide or potassium hydroxide is that the demand for these products, especially local demand, may exceed that for the chlorine by-product. If, for example, potassium hydroxide is produced in connection with fertilizer manufacture, the handling, packaging and shipment of by-product chlorine may, on the scale produced, by relatively uneconomical. Thus, a need exists in certain localities for processes producing alkali metal hydroxides more efficiently alone, without the production of chlorine.